This invention relates to transformers and more particularly to transformers with a disc wound coil.
As is well known, a transformer converts electricity at one voltage to electricity as another voltage, either of higher or lower value. A transformer achieves this voltage conversion using a primary coil and a secondary coil, each of which is wound on a ferromagnetic core and comprise a number of turns of an electrical conductor. The primary coil is connected to a source of voltage and the secondary coil is connected to a load. The ratio of turns in the primary coil to the turns in the secondary coil (“turns ratio”) is the same as the ratio of the voltage of the source to the voltage of the load. Two main winding techniques are used to form coils, namely layer winding and disc winding. The type of winding technique that is utilized to form a coil is primarily determined by the number of turns in the coil and the current in the coil. For high voltage windings with a large number of required turns, the disc winding technique is typically used, whereas for low voltage windings with a smaller number of required turns, the layer winding technique is typically used.
In the layer winding technique, the conductor turns required for a coil are wound in one or more concentric conductor layers connected in series, with the turns of each conductor layer being wound side by side along the axial length of the coil until the conductor layer is full. A layer of insulation material is disposed between each pair of conductor layers. Axially-extending air ducts may also be formed between pairs of conductor layers. In U.S. Pat. No. 7,023,312, pre-formed cooling ducts are inserted between conductor layers during the winding of a coil.
In the disc winding technique, the conductor turns required for a coil are wound in a plurality of discs serially disposed along the axial length of the coil. In each disc, the turns are wound in a radial direction, one on top of the other, i.e., one turn per layer. The discs are connected in a series circuit relation and are typically wound alternately from inside to outside and from outside to inside so that the discs can be formed from the same conductor. An example of such alternate winding is shown in U.S. Pat. No. 5,167,063.
In a transformer with a conventional disc-wound coil, the capacitance between the discs is fairly low in comparison with the capacitance between the discs and ground. As a result, when the transformer is subjected to a steep wave front impulse or transient voltage, such as may occur as a result of a lightning strike, a significant non-linear voltage distribution occurs along the axial length of the coil with a very high voltage gradient appearing at the first few turns adjacent the high voltage end. This high voltage gradient produces significant local dielectric stresses.
In order to increase series capacitance and improve impulse voltage distribution, the discs may be interleaved, i.e., the turns of adjacent discs may be interleaved. An example of a transformer with interleaved discs is shown in U.S. Pat. No. 3,958,201. Forming interleaved discs, however, is complicated and decreases the free space between discs, which adversely affects cooling.
It would therefore be desirable to provide a transformer with disc-wound coils, which has improved impulse voltage distribution and cooling. The present invention is directed to such a transformer and a method for manufacturing such a transformer.